A typical airship is comprised of a hull which can be either non-rigid, semi-rigid, or rigid. A non-rigid airship normally comprises a pressurized, gas tight fabric envelope constructed of several layers of impregnated cloth, and it relies upon internal pressure to enable it to endure compressive or bending loads. A rigid airship comprises a hull normally constructed of metal or wood which is covered with a doped fabric to form the airship's envelope. A rigid airship does not depend on internal pressure for strength and stability. Semi-rigid airships contain the features of both rigid and non-rigid airships.
An airship achieves most of its lift from the lighter-than-air gas contained within its envelope. The two most frequently used gases are hydrogen and helium, although helium is more commonly used because it is readily available, nonflammable and inert. The mobility of an airship comes from its propulsion system which may include an internal combustion engine, a fuel source, and a propeller and rudder system. One of the major advantages of an airship is its ability to remain aloft for extended periods of time without the need for returning to the earth for refueling as is required by conventional aircraft. This ability to remain aloft for extended periods of time results from the fact that an airship's loft is achieved primarily by the helium or hydrogen contained within its envelope, not from any lift resulting from its propulsion system. While an airship has the ability to remain aloft for an extended period of time, such time period is not unlimited, but rather is limited by the airship's fuel capacity.